An important system used in many copiers and printers is an imaging drum for holding and moving print media in a known relationship relative to the imaging or printing elements. One of the key problems associated with such drums involves mounting the print media in a fixed position on the drum, and monitoring the accuracy of the mounting process in order to detect paper jams and improper mounting positions.
In the prior art these problems have been dealt with in several different ways. For example, expensive and complicated mechanical feed systems have been developed to load the media onto the drum and to hold the media in juxtaposition with the drum. See for example, U.S. Pat. No. 4,025,175, issued May 24, 1977 entitled "Electrophotographic Copying Apparatus Having a Dual Cam Synchronizing Mechanism" by Yokozawa et. al., assigned to Iwatsu Electric Co., Ltd., and laid open Japanese Utility Model Applications, Nos. 75345/74 and 56826/78. Similarly, vacuum systems which in some instances are simpler and less expensive than these mechanical systems have been used to load the media onto the drum and to hold the media in place on the drum. Even in vacuum systems, however, an automated clamping system is sometimes used to hold down the leading edge of the media while the drum is rotated to bring the rest of the media into contact with the drum. Typical examples of some vacuum loading systems are illustrated by U.S. Pat. No. 4,345,752, issued Aug. 24, 1982, entitled "Sheet Transfer Apparatus," by Nakamura et. al., assigned to Tokyo Shibaura Denki Kabushiki Kaisha; U.S. Pat. No. 3,865,364 issued Feb. 11, 1975 entitled "Anti-Curl Sheet Feeding Apparatus" by Robert A. Sterner, assigned to FMC Corporation; and U.S. Pat. No. 4,310,151 issued Jan. 12, 1982, entitled "Sheet Feeding Apparatus" by Sakae Fujimoto, assigned to Ricoh Company, Ltd.
In all of these systems, it is often difficult and expensive to monitor the mounting process to detect media jams and improper mounting. Hence, it is not unusual for manufacturers to rely almost entirely on the operator to notice that the media is not being fed or that copies are skewed, rather than to use machine diagnostics and monitoring systems. ln automated systems, however, an operator may not be available to provide for such diagnostics. Hence some systems have been developed which directly monitor the mounting process. For example, a typical approach is to use a system of photo-detectors to determine the locations of the edges of the media relative to particular points in the system at different times. The general result is that even for relatively few monitoring points, the system can be quite sophisticated and expensive, and the monitoring apparatus itself can be prone to failure. In addition, for vacuum operated systems, such monitoring is especially important due to the fact that changes in air density, for example due to changes in geographical elevation, temperature and humidity, can have dramatic effects on the vacuum characteristics and hence serious deleterious consequences on the media handling ability of the system.
What is needed is a simple, inexpensive media handling system, such as a vacuum system, which has a built-in monitoring function and which is adaptive to changes in air density.